Materials Handbook (Handbook) 15th Edition
ABLATIVES. Materials used for the outward dissipation of extremely high heats by mass removal. Their most common use is as an external heat shield to protect supersonic aerospace vehicles from an excessive buildup of heat caused by air friction at the surface. The ablative material must have a low thermal conductivity in order that the heat may remain concentrated in the thin surface layer. As the surface of the ablator melts or sublimes, it is wiped away by the frictional forces that simultaneously heat newly exposed surfaces. The heat is carried off with the material removed. The less material that is lost, the more efficient is the ablative material. The ablative material also should have a high thermal capacity in the solid, liquid, and gaseous states; a high heat of fusion and evaporation; and a high heat of dissociation of its vapors. The ablative agent, or ablator, is usually a carbonaceous organic compound, such as a phenolic plastic. As the dissociation products are lost as liquid or vapor, the char is held in place by the refractory reinforcing fibers, still giving a measure of heat resistance. The effective life of an ablative is short, calculated in seconds per millimeter of thickness for the distance traveled in the atmosphere.
Single ablative materials seldom have all the desirable factors, and thus composites are used. Phenolic or epoxy resins are reinforced with asbestos fabric, carbonized cloth, or refractory fibers, such as asbestos, fused silica, and glass. The refractory fibers not only are incorporated for mechanical strength, but also have a function in the ablative process, and surface-active agents may be added to speed the rate of evaporation. Another composite, polyarylacetylene (PAA) reinforced with carbon fiber fabric, proved superior to carbon-reinforced phenolic in tests to develop an alternative ablative and insulative material for nozzle components of solid rocket motors. Favoring the PAA is its high (90%) char yield, lower weight loss and erosion, greater moisture resistance, and more stable ablation. Ablative paint, for protecting woodwork, may be organic silicones which convert to silica at temperatures above 2000Â°F (1093Â°C).
Metals can resist temperatures higher than their melting point by convection cooling, or thermal cooling, which is heat protection by heat exchange with a coolant. Thus, tungsten can be arc-melted in a copper kettle which is cooled by circulating water. The container metal must have high thermal conductivity, and the heat must be quickly carried away and stored or dissipated. When convection cooling is difficult or not possible, cooling may be accomplished by a heat sink. Heat-sink cooling depends on the heat absorption capability of the structural material itself or backed up by another material of higher heat absorption. Copper, beryllium, graphite, and beryllium oxide have been used. A heat-sink material should have high thermal conductivity, high specific heat and melting point, and for aerodynamic applications, a low specific gravity.
ABRASIVES. Materials used for surfacing and finishing metals, stone, wood, glass, and other materials by abrasive action. The natural abrasives include the diamond, emery, corundum, sand, crushed garnet and quartz, tripoli, and pumice. Artificial abrasives, or manufactured abrasives, are generally superior in uniformity to natural abrasives, and are mostly silicon carbide, aluminum oxide, boron carbide, or boron nitride, marketed under trade names. Artificial diamonds are also now being produced. The massive natural abrasives, such as sandstone, are cut into grinding wheels from the natural block, but most abrasive material is used as grains or built into artificial shapes. For industrial grinding, artificial abrasives are preferred to natural abrasives because of their greater uniformity. Grading is important because uniform grinding requires grains of the same size. The abrasive grains are used as a grinding powder; are made into wheels, blocks, or stones; or are bonded to paper or cloth. Abrasive cloth is made of cotton jean or drills to close tolerances of yarns and weaves, and the grains are attached with glue or resin. But the Fabricut cloth of 3M is an open-weave fabric with alumina or silicon-carbide grains of 100 to 400 mesh. The open weave permits easy cleaning of the cloth in an air blast. Abrasive paper has the grains, usually aluminum oxide or silicon carbide, glued to one side of 40- to 130-lb kraft paper. The usual grain sizes are No. 16 to No. 500.
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|May 30, 2020|